Invoice for the previous periods
If I understand it right, services are always invoiced in advance. If I set the due date to 1.6.2013 for monthly billing, invoice will be generated for the period from 1.6.2013 to 30.6.2103 on 1.6.2013. So the client will have to pay for the following month.
In my case clients use to pay after, what means they expect the invoice for the period from 1.6.2013 to 30.6.2103 on 1.7.2013. They can enable and use all the services they want and pay for them at the end of the month